The present invention relates to an electrode array for use with a cochlear stimulator. More particularly, the present invention relates to the method of making an electrode array that: (1) uses geometrically shaped and/or treated electrode contacts to better steer or focus the electrical stimulation current to desired target tissue; and (2) uses coatings of a selected drug(s) or compound(s) on the electrical contacts (or spread over the entire electrode array) to promote a desired therapeutic treatment of the tissue in the vicinity of the electrode array, e.g., to inhibit growth of fibrous tissue or bone tissue, to promote healing, to prevent neural degeneration, and/or to promote neural regeneration.
Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information does reach the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or the destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. These people are unable to derive any benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because their mechanisms for transducing sound energy into auditory nerve impulses have been damaged. Thus, in the absence of properly functioning hair cells, there is no way auditory nerve impulses can be generated directly from sounds.
To overcome sensorineural deafness, there have been developed numerous cochlear implant systems—or cochlear prosthesis—which seek to bypass the hair cells in the cochlea (the hair cells are located in the vicinity of the radially outer wall of the cochlea) by presenting electrical stimulation to the auditory nerve fibers directly, leading to the perception of sound in the brain and at least partial restoration of hearing function. The common denominator in most of these cochlear prosthesis systems has been the implantation into the cochlea of electrodes which are responsive to a suitable external source of electrical stimuli and which are intended to transmit those stimuli to the ganglion cells and thereby to the auditory nerve fibers.
A cochlear prosthesis operates by direct electrical stimulation of the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity in such nerve cells. In addition to stimulating the nerve cells, the electronic circuitry and the electrode array of the cochlear prosthesis performs the function of the separating the acoustic signal into a number of parallel channels of information, each representing the intensity of a narrow band of frequencies within the acoustic spectrum. Ideally, each channel of information would be conveyed selectively to the subset of auditory nerve cells that normally transmitted information about that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from high frequencies at the basal end of the cochlear spiral to progressively lower frequencies towards the apex. In practice, this goal tends to be difficult to realize because of the anatomy of the cochlea.
Over the past several years, a consensus has generally emerged that the scala tympani, one of the three parallel ducts that, in parallel, make up the spiral-shaped cochlea, provides the best location for implantation of an electrode array used with a cochlear prosthesis. The electrode array to be implanted in this site typically consists of a thin, elongated, flexible carrier containing several longitudinally disposed and separately connected stimulating electrode contacts, perhaps 6-30 in number. Such electrode array is pushed into the scala tympani duct to a depth of about 20-30 mm via a surgical opening made in the round window at the basal end of the duct. During use, electrical current is passed into the fluids and tissues immediately surrounding the individual electrode contacts in order to create transient potential gradients that, if sufficiently strong, cause the nearby auditory nerve fibers to generate action potentials. The auditory nerve fibers arise from cell bodies located in the spiral ganglion, which lies in the bone, or modiolus, adjacent to the scala tympani on the inside wall of its spiral course. Because the density of electrical current flowing through volume conductors such as tissues and fluids tends to be highest near the electrode contact that is the source of such current, stimulation at one contact site tends to activate selectively those spiral ganglion cells and their auditory nerve fibers that are closest to that contact site. Thus, there is a need for the electrode contacts to be positioned as close to the ganglion cells as possible. This means, in practice, that the electrode array, after implant, should preferably hug the modiolar wall, and that the individual electrodes of the electrode array should be positioned on or near that surface of the electrode array which is closest to the modiolar wall so that the stimulation current flowing from or to the electrode contacts can effectively stimulate the ganglion cells.
In order to address the above need, it is known in the art to make an intracochlear electrode array that includes a spiral-shaped resilient carrier which generally has a natural spiral shape so that it better conforms to the shape of the scala tympani. See, e.g., U.S. Pat. No. 4,819,647. The '647 U.S. patent is incorporated herein by reference.
Applicant's related patent applications, referenced above, also disclose a preferred approach for achieving an electrode array that hugs the modiolar wall of the scala tympani. The teachings of those patent applications, repeated in large part herein, are also applicable to the present invention.
The geometry of the electrode contacts provided by traditional cochlear electrodes, i.e., the shape of the exposed electrode contact, has heretofore been determined primarily by the need to place the electrode in certain locations within the tissue where the electrode is to be implanted. Disadvantageously, the concentration of current densities associated with such traditionally-shaped electrodes has not always been optimized to concentrate current flow in the target tissue area. Hence, there is a need for a cochlear electrode array wherein the electrode contacts themselves are designed to better steer or focus the stimulation current to the target tissue.
One exception to the traditional approach of electrode contacts is disclosed in U.S. Pat. No. 5,649,970, incorporated herein by reference. However, the approach shown in the '970 patent, while quite effective at achieving its intended purpose, is difficult and expensive to manufacture.
Further, it is noted that when the electrode array is first inserted into the scala tympani, and thereafter, there may be a need to treat the tissue in the surrounding area with an appropriate drug or other compound in order to, e.g., inhibit the growth of fibrous tissue, inhibit the growth of bone tissue, promote healing, prevent neural degeneration, and/or promote neural regeneration. Other than applicant Kuzma's provisional patent application Ser. No. 60/101,942, filed Sep. 25, 1998, referenced above, applicants are not aware of any cochlear electrode designs that address this need to apply therapeutic drugs or other compounds to the target cochlear tissue.
It is thus evident that improvements are still needed in cochlear electrodes, particularly to facilitate assembling an electrode so that the electrode contacts are on the medial side of the electrode array, to better assure that the electrode assumes a close hugging relationship with the modiolus once implantation of the electrode has occurred, to enhance the ability of the electrode to better direct or focus the current density flowing to/from the electrode contacts at the target tissue, and/or to provide ways to apply therapeutic drugs or other compounds to the target tissue at the same time that electrical stimulation is provided to the target tissue.